In many wireless antenna applications it is desirable to receive or transmit signals from a wide variety of possible angles. However, the radiation pattern of an antenna element is never completely omni-directional, as there is always a direction from which an antenna receives less power than its optimal direction.
Various attempts have been made to combine monopole and dipole antennas so as to create composite antennas that can transmit or receive from more directions with a more even power distribution. The ideal is generally to create a hemispherical radiation pattern for an antenna over a ground plane. However, the combination of a single monopole and dipole do not produce a radiation pattern that is very hemispherical as there are multiple local minima. In addition, collocation of the monopole and dipole is generally a problem and many previous attempts to combine monopoles and dipoles are sub-optimal because they are not accurately collocated.
The applicant's own PCT publication number WO2015107473, which is incorporated by reference in its entirety herein, discloses two embodiments of a composite antenna. The two composite antenna embodiments disclosed combine a monopole and dipole antenna to form a composite antenna that can transmit or receive from more directions with a more even power distribution.
The second antenna embodiment disclosed in WO2015107473, which has sector-shaped dipole arms and a conical extension of a conductive tube, suffers from two drawbacks. A first problem with this antenna is that there is an impedance mismatch between the antenna and the signal transmission lines for one of the excitation modes, namely mode TEM4, in a frequency range of interest. Excitation mode TEM4 is a mode which results in out of phase excitation between adjacent dipole arms, resulting in power radiated between adjacent dipole segments. The impedance of the antenna for this excitation mode is poorly matched compared with the other three excitation modes (TEM1, TEM2 and TEM3) in a frequency range of interest. A poor impedance match results in power being reflected, either reflected back along the signal transmission lines when the antenna is used as a transmitter, or reflected away from the antenna when the antenna is used as a receiver.
A further problem with the disclosed antenna is that fields are induced between inner surfaces of the conical extension, resulting in unwanted interference.
The invention aims to address these and other shortcomings, at least to some extent.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.